<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>


<script>
    //let a =prompt("输入整数")
    //let sum = 0;
    //for(let b = 1; b <= a; b++ ){
    //    if(b%2==0){
    //        sum += b;
    //    }
    //}
    //alert(sum);

    var arr = [1,2,3]
    arr[10] = 10;
    alert(arr)
</script>
</body>
</html>